1. Field of the Invention
The present invention lies in the area of printed circuit board (PCB) component assembly and manufacturing.
2. Description of the Related Art
In radio frequency (RF) products, certain components must be positioned to be attached through the circuit board and onto a mounting plate (also called a casting) that backs the circuit board within extremely tight positioning tolerances. The proposed invention provides an economical method for eliminating variances in positioning mechanics that occur when human operators position components manually.
Many RF assemblies consist of a printed circuit board fastened onto a mounting plate. This plate serves as both a ground reference and a thermal heat sink. Although some of the RF components are mounted to the surface of the PCB via the conventional surface mount procedures, others are mounted to both the PCB and to the mounting plate, again for reasons of ground references and thermal conductivity. RF transistors, for example, are typically mounted to both the PCB and the mounting plate. In either case, the component is mounted in a component cutout found on the PCB. The mounting plate itself may contain a component cutout if the component is to be mounted to the mounting plate.
Because the frequencies involved are high (for this invention, this means over about 250 MHz), the length of the RF waves on the circuit board are short (several inches). For RF circuits the correct performance of the assembly depends on accurately locating the components and circuit board to a small fraction of the wavelength, in accordance with the design intent. If either the components or the PCB are located incorrectly relative to each other, both electrical (RF) and thermal performance will be degraded. If the misalignment is severe, in addition to degraded performance, reliability may be significantly impaired to the extent of catastrophic failure when the unit is turned on.
At relatively low frequencies and power levels (a few hundred MHz and a few watts) an alignment tolerance of 0.010xe2x80x3 to 0.025xe2x80x3 would generally be adequate. The exact tolerance would be determined by CAD simulation or through testing. As the frequency and/or the power levels increase, the tolerance for positioning the PCB on the plate, as well as the tolerance for positioning the components on the PCB, decreases. At a frequency of 1 GHz and a power level of tens of watts, the tolerance allowed for correct performance may be as small as 0.001xe2x80x3 to 0.005xe2x80x3. Consistency of position from one assembly to the next is also important in order to maintain the same performance of all assemblies.
Tolerances this small are difficult, time-consuming, or expensive to achieve on a production basis. Among the problems encountered is that a component is designed to fit into a cutout of the PCB, but the cutout provides for too much spacing. The component could be placed into the cutout but still not be aligned precisely enough for the PCB. More importantly, the cutouts on the PCB and the mounting plate are capable of being cut to the same dimensions in a production environment but are not generally capable of being consistently positioned while in production. Thus, any given component is subject to incorrect positioning both relative to its cutout and relative to other components.
The prior art solutions for properly positioning components generally involve manual techniques: human operators currently place the component into the cutout in the circuit board and casting, manually align the component with the proper edge of the cutout, then while holding the component in place with one hand, obtain screws and a screwdriver, position the screws, and drive them into place. This method, while simple, runs the risk of damaging the component because the operator must exert pressure to hold the component. For example, the operator must exert pressure to keep the component in position into the PCB aperture, as well as when the operator aligns the component to the edge of the PCB aperture.
More importantly, operator placement of the component is rough and repeatable precision is difficult to achieve with the human eye. Although this could be solved with a machine vision system, such a system may be prohibitively expensive to implement. Finally, the component is subject to motion while the operator disengages one hand to obtain screws or a screwdriver, or the component may move when the screws are tightened.
Briefly described, the invention comprises a method and apparatus for positioning electronic components prior to their assembly onto a printed circuit board and backing plate. The preferred embodiment of the invention consists of a floating nest that fits in an aperture on a top plate. The top plate rests on top of the printed circuit board and mounting plate and the nest itself fits loosely in the top plate aperture to allow for positioning.
In the preferred embodiment, the operator places the component in the nest and wiggles the nest until the walls of the nest engage the edge of the PCB aperture, aligning the nest with the PCB aperture. External springs permit the nest to be wiggled into position, and springs on the nest""s internal walls center the component relative to the nest. The component can then be permanently fastened to the board and the mounting plate.
In an alternative embodiment, the floating nest is enhanced to allow for use of a pushing force in aligning the component. An eccentric cam is used to move a push plate that in turn applies pressure to a leaf spring that abuts the component to be positioned. In this embodiment, the operator places the component in the nest and then pushes the component until the component is pushed into proper position. The enhanced nest permits the operator to precisely position and hold the component towards one edge of the nest, rather than centering the component.